1. Field of the Invention
The present invention relates to telecommunication signal switching equipment and is more particularly concerned with broadband signal switching equipment.
2. Description of the Prior Art
Modern developments in telecommunication engineering lead to integrated services message transmission and switching systems for a narrowband communication services and broadband communication services, which provide light waveguides as transmission media in the area of the subscriber lines by way of which narrowband communication services, in particular 64 kbit/s digital telephony, are guided as well as broadband communication services, particularly 140 Mbit/s video telephony, whereby, in the switching stations, also (preferably with joint control units) narrowband signal switching devices and broadband signal switching devices can be provided in a side-by-side manner, as disclosed in the German patent 24 21 002, corresponding to U.S. Pat. No. 3,980,831, fully incorporated herein by this reference.
As is known, for example from the European application 0 262 479, corresponding to U.S. Pat. No. 4,801,936, also fully incorporated herein by this reference, broadband signal switching equipment has a switching point matrix constructed in field effect transistor (FET) technology whose switching elements are composed of, respectively, one switch transistor which is charged, at a control electrode, with a through connect or blocking signal and connected to the appertaining matrix output line with a main electrode, whereby the switching elements respectively have an auxiliary transistor connected in series with the switch transistor, the auxiliary transistor being connected with its control electrode to the appertaining matrix input line, and its main electrode, not facing the serial connection, being connected to the one terminal of the operating voltage source by way of a sampling transistor, the other terminal of the operating voltage source being connected to the respective matrix output line via a precharge transistor, and whereby the precharge transistor and the sampling transistor are charged oppositely respectively at their control electrodes with a selection clock for the switching matrix array dividing a bit switch through interval into a precharge phase and the actual through connect phase, so that in each precharge, given a blocked sampling transistor, the matrix output line is charged, via the precharge transistor, at least approximately to the potential prevailing at the mentioned other terminal of the operating voltage source. This known broadband signal switching equipment, which may have sampling transistors for individual switching elements or sampling transistors for individual matrix input lines or matrix output lines requires, for the selection of the sampling transistors, its own clock pulse lines extending through the switching point matrix causing a corresponding space requirement and a corresponding capacitive stress on the matrix output lines; in order to guarantee sufficient interference protection, clock distribution and couplings between matrix input lines and matrix output lines require sufficiently large signal amplitudes on the matrix output lines which is connected with a relatively high power consumption.
In the European patent application 88 11 2908.4, corresponding to U.S. Ser. No. 390,838, filed Aug. 8, 1989, now abandoned, a broadband signal switching equipment is disclosed which has a crosspoint matrix constructed in FET technology whose inputs can respectively be provided with an input drive circuit, whose outputs are respectively provided with an output amplifier circuit, whose switching elements, respectively controlled by a holding memory cell, are constituted respectively by a serial connection of a switch transistor charged at its control electrode with a through connect or block signal and an input transistor connected, via its control electrode, to the appertaining matrix input line, the serial connection between connected to the appertaining matrix output line with the main electrode of the one transistor facing away from the serial connection, and whereby the matrix output line is connected to a precharge potential source via a precharge circuit, which is connected, with an unlocking input to the clock signal line of a precharge clock signal defining the precharge phase of a bit through connect interval divided in such a precharge phase and rest bit through connect interval, so that the matrix output line is charged to a precharge potential in each precharge phase, has been suggested, which provides a swing limitation of the signal on the output lines of the switching equipment. For that purpose, the main electrode which faces away from the serial connection of the other transistor of each switching element is connected to the one terminal of the operating voltage source via a transistor individually provided for the matrix output line that is connected, at its control electrode, to the output of an amplifier circuit individually provided for the matrix output line; given a signal state alteration at the output of the output amplifier circuit, this transistor is blocked and, by that, a further charge reversal of the output line is avoided with which the signal swing on the output line is limited. This signal swing limitation is, however, still incomplete insofar as, after the transistor that exists exclusively for the matrix output lines is blocked, a charge compensation still takes place between the line (connecting the switching elements leading to this matrix output line to the transistor that exists exclusively for the matrix output line) and the matrix output line, so that the output signal level changes even further.